1. Field of the Invention
The present invention relates to a cooling apparatus in which streams of a coolant are made to blow against an object to cool as jet streams.
2. Description of Related Art
A cooling apparatus is generally used for cooling various electronic devices with generation of heat. In particular, for cooling an electronic device with a great generation of heat such as a power amplifier of a television transmitter, used is a cooling apparatus in which a coolant such as air is jetted through nozzles and made to blow directly against an object to cool so that the object is cooled by compulsory heat exchange.
As an example of a related art cooling apparatus, Japanese Patent Unexamined Publication No. Hei 1-175298 (published in Japan on Jul. 11, 1989) discloses a cooling structure for an electronic circuit. The structure includes an air pipe provided with a plurality of nozzles for spouting air, which flows in the pipe, toward an element with a little generation of heat such as a memory IC.
The structure disclosed in this publication includes a fixing board which is attached to a frame so as to be opposite to a circuit board, a cooling board which is in contact with an element with a great generation of heat through an elastic sheet with a good heat conductivity attached to the fixing board, and is provided in its inside with a flow passage for allowing a liquid coolant to flow, and the air pipe which is attached to the fixing board and provided with the nozzles for spouting air, which flows in the pipe, toward the element with a little generation of heat. The element with a little generation of heat is cooled by a so-called collision cooling method.
Another related art cooling apparatus will be described further.
FIG. 5 is a cross-sectional side view of a related art cooling apparatus.
In the cooling apparatus shown in FIG. 5, air sucked through a blower 4 is introduced into a connecting chamber 10 as compressed air 7. After introduced into a duct 2, the compressed air 7 is horizontally jetted as cooling air streams 6 through nozzles 5 each bored through a nozzle plate 9 of the duct 2, and the air is made to blow against a heat sink 1a of a power amplifier 1 as the object to cool so that the object is cooled by compulsory heat exchange.
When such a related art cooling apparatus as described above is applied to an electronic device with a great generation of heat, however, the following problems arise.
As the first problem, a high-power television transmitter is received in a receiving rack, the heat sink 1a of the power amplifier 1 of the television transmitter is opposite to a plurality of nozzles 5, and the duct 2 has the same horizontal cross section throughout its vertical length. As a result, the quantities of the cooling air streams 6 jetted through the nozzles 5 are uneven in the manner that the lower the nozzle 5 is, the more the quantity is. This causes an inclination in the temperature distribution of the heat sink 1a, and the lower portion of the heat sink 1a is fully cooled but the upper portion of it is hardly cooled.
As the second problem, a noise called jet noise is generated at the nozzles 5 when the cooling air streams 6 are jetted through the nozzles 5. Because this jet noise is a random noise consisting of frequencies in a broad range, it is a mere noise which is heard as a rumble, and does not give so unpleasant feeling to a human in the neighborhood. But in case that the horizontal distance in the duct 2 along the direction in which the cooling air streams 6 are jetted, that is, the horizontal distance l between the nozzle plate 9 and the opposite plate 3 coincides with a resonance distance l.sub.0 which is equal to the wavelength of a specific peak frequency in the jet noise, a resonance occurs to increase the energy of the noise. A big sound like a whistle is therefore generated and it gives an unpleasant feeling to a human in the neighborhood.
FIG. 6A is an enlarged partial side view in section for illustrating the principle of occurrence of the resonance in the cooling apparatus of FIG. 5. FIG. 6B is an enlarged view of a nozzle 5 of FIG. 6A. FIG. 7 is an enlarged partial side view in section for illustrating the principle of occurrence of the resonance in the cooling apparatus of FIG. 5.
In case that the horizontal distance l between a nozzle 5 (for example, a point P in FIG. 6A) of the nozzle plate 9 and a point (for example, a point A in FIG. 6A) at which a horizontal extension of the nozzle 5 intersects the inside surface of the opposite plate 3 is equal to a resonance distance l.sub.0 at which a standing wave W of a cooling air stream 6 is generated in the duct 2, that is, (the horizontal distance l)=(a resonance distance l.sub.0), a resonance occurs in the duct 2 due to the standing wave W of the cooling air stream 6 and so a whistle sound is generated.